Take in account the number of deletes in the LogMergePolicy

[fulmicoton]

The current LogMergePolicy does not take in account deletes today. As a result, some bad behavior may appear
under the following natural workload.

• Documents are inserted continually.
• Documents are deleted continually. For instance as they get too old.

As merge are occuring one very large segment may appear.
It might take a very long time for segment of an equivalent size to appear, so that this segment virtually never get merged.
Yet, all or almost all of its docs might be deleted.

It would be beneficial to refine the heuristics of the log merge policy to take in account the ratio of delete documents as well.

[fulmicoton]

Assigned informally to @blitzerr

[fulmicoton]

@guilload Can you mentor @blitzerr on this ticket?